The present invention relates to a disk wheel for a motor vehicle which consists of a rim section and a disk section and, more particularly, to a steel disk wheel which is formed of a steel sheet.
A motor vehicle is generally furnished with a disk wheel which consists of a hollow cylindrical rim section for mounting a tire, and a disk section for mounting the wheel to a hub. A disk wheel of this kind is often formed of a steel sheet.
It has been customary to form the rim section and the disk section of a steel disk wheel independently of each other and put them together by fitting the disk section in the inner periphery of the rim section and connecting them by welding. The rim section is produced by roll-forming a steel sheet while the disk section is produced by press-forming a steel sheet. Hence, the rim section has the same thickness in both of its inner rim portion and outer rim portion.
The disk section includes a disk surface which is usually offset toward the outer rim portion with respect to the center of the rim width, so that the distance between an extension of a king pin shaft and the center of the tread of a tire, i.e., scrub distance may be maintained short. This necessarily causes the disk surface to be positioned at a substantial distance from the inner rim portion, resulting that a considerable bending moment acts on the inner rim portion due to a load imparted from a tire. The inner rim portion, therefore, has to be provided with greater rigidity than the outer rim portion so as to withstand such bending moments.
However, when it comes to a prior art rim section having an inner rim portion and an outer rim portion which are formed integrally by roll-forming, the thickness cannot be varied from the inner rim portion to the outer rim portion. The only implementation available to enhance the rigidity of the inner rim part is increasing the thickness of the whole rim section at the cost of weight.
To accurately fit and secure a tire to the rim section, the rim section is provided with flange surfaces for making close contact with the sides of the beads of a tire and bead seat surfaces for making close contact with the inner circumference of the same. A prerequisite with such a rim section is that the flange surfaces and the bead seat surfaces, especially those of the inner rim portion which is remote from the disk surface, be maintained in a true circular configuration with respect to the center of rotation of the wheel. This requisite cannot be readily satisfied with the prior art fitting type assembly of the rim section and disk section. For example, when the disk section is fitted in the rim section in an inclined position relative to the center plane of the rim width, the flange surfaces and bead seat surfaces become eccentric to the center axis of the disk section, i.e., the axis of rotation of the wheel.
Moreover, the accuracy of those flange surfaces and bead seat surfaces is critically effected by that of a roll-forming or press-forming machine. Hence, it is difficult for the accuracy of flange surfaces and bead seat surfaces to be enhanced to a satisfactory degree. It has therefore been practiced to precision-finish the flange surfaces and bead seat surfaces of a steel disk wheel by cutting after the assemblage of the wheel.
A problem with such a cutting-after-assemblage scheme is that because the rim section of a steel disk wheel is formed of a thin sheet, the flange surfaces and bead seat surfaces cannot be precisely cut with ease. Especially, the flange surfaces are apt to be deformed sideways and, thereby, oscillate when a tool is applied thereto, because they are located at the opposite free ends of the rim section and, moreover, extend substantially vertically to the axis of the rim section. This brings about fine undulation on the resultant flange surfaces. The undulation on the tire support surfaces would not only make it difficult to securely hold a tire but also develop clearance between a tire and the rim section to lower sealability and, thereby, aggravate leakage of air from the tire.
When the circularity of a wheel is implemented with cutting only, it often occurs that a wheel has to be cut by a considerable amount. Such would reduce the thickness and, therefore, the mechanical strength of the rim section.
Another drawback with the welded rim and disk assembly is that the welded area of the assembly is exposed to the atmosphere and, therefore, easy to gather rust due to deposition of water.